The "Hering-Breuer" lung volume reflexes are mediated by afferent fibers of the vagus nerves and act upon the central ventilatory control mechanism to cause an inhibition of ventilation upon lung inflation and an excitation of ventilation upon deflation. Although the earliest investigators believed that these reflexes provided a clear mechanism whereby rate and tidal volume of breathing could be adjusted, a few investigators have recently questioned the role of either of the volume reflexes in breathing control. The purpose of the present study is to define nervous system mechanisms which may influence the expression of "Hering-Breuer" reflexes in relation to the ventilatory pattern during normal and chemostimulated (with hypoxia, hypercapnia, or asphyxia) ventilation. The preparation to be used is the Virginia opossum, a marsupial mammal born at a very primitive stage of embryological sophistication. Development of ventilatory pattern as a possible function of each of the reflexes will be investigated as the animal matures. Lung inflations and deflations as well as measurement of ventilation breathing various gases will be performed using pressure plethysmography. Analysis of the data will consist of relating reflex strength to the afferent stimuli (arterial PO2 and PCO2) and the efferent response (tidal volume, respiratory frequency, minute ventilation). In further work, central and peripheral nervous system lesions, as well as a sleep study, will be performed in an attempt to provide functional or anatomical basis for any developmental effects. Any consistent relationship found between reflex strength and efferent response during maturation will implicate that reflex as being necessary to an appropriate ventilatory response. The study may, in this way, determine if either of the "Hering-Breuer" reflexes has a physiological role in the control of breathing.